Since 5b3fb024be, the main window is only shown when not launched
with a valid UUID. As GtkDialog isn't meant to be used standalone,
we currently trigger a (harmless but annoying) warning in case
the main window isn't shown; we can avoid the warning by setting
up the preference dialog manually instead of using the GtkDialog
convenience class.
https://bugzilla.gnome.org/show_bug.cgi?id=781545